The present invention is directed to novel carbodiimide compounds and to their method of preparation. Such compounds find use in ambient cure automotive paint compositions.
Carbodiimide compounds are known to react with a carboxyl group at ambient temperatures to form an N-acyl urea. The reaction can be represented by ##STR1##
For this reason, this chemistry has been employed to cross-link link or cure carboxyl containing coating compositions at ambient temperatures. Commercial polycarbodiimide cross-linkers are available and typically are based on aliphatic and cycloaliphatic isocyanates.
According to one aspect of the present invention, an improved coating composition containing the novel carbodiimide compounds with improved drying, improved pot life, ambient curing and durability without the known harmful effects of an isocyanate cure are disclosed.
Any substrate material can be coated with the coating composition according to the present invention. These substrate materials include such things as glass, metal, ceramics, paper, wood and plastic. Coating systems formed using the present invention are particularly adapted for metal substrates and specifically as an automotive refinish system. The substrates may be uncoated material or can be primed. The substrate may also be coated with paint products applied at the time of manufacture. The coating composition can be applied using conventional spray equipment or high volume/low pressure spray equipment.
One of the major difficulties found in this technology and especially in solvent based coating systems is that the reaction tends to take place too rapidly, rendering a two-component coating system with an unusably short pot-life. Some strategies have been developed to retard the carbodiimide/carboxyl reaction to improve pot-life such as salting with tertiary amines, using low acid value coreactants or introducing latent reactive groups. These strategies, however, will either compromise the ultimate film properties, not succeed in sufficient pot-life extension, or both. A slower reacting, light colored, carbodiimide species is therefore desired to overcome the problems of the prior art without special processing or by-products that require disposal during manufacture.